FIG. 1 shows a conventional asynchronous boost converter 100, in which an inductor L1 is connected between a power input VIN and a phase node LX, a power switch 104 is connected between the phase node LX and ground GND, a diode D1 is connected between the phase node LX and a power output VOUT, a capacitor COUT is connected between the power output VOUT and ground GND, and a controller 102 provides a pulse width modulation (PWM) signal to switch the power switch 104 to convert the input voltage VIN to an output voltage VOUT. In the controller 102, voltage divider resistors R1 and R2 divide the output voltage VOUT to generate a feedback voltage VFB, an error amplifier 114 generates an error signal EA according to the difference between the feedback voltage VFB and a reference voltage VREF, a comparator 110 generates a signal COMP according to the error signal EA and a ramp signal RAMP, a flip-flop 108 generates the PWM signal according to the signal COMP and a clock CLK provided by an oscillator 106, and a gate driver 112 switches the power switch 104 in response to the PWM signal, so as to regulate the inductor current IL1.
FIG. 2 is a waveform diagram showing the inductor current IL1 and some corresponding signals in the converter 100 of FIG. 1, in which waveform 150 represents the PWM signal, waveform 152 represents the phase node voltage LX, and waveform 154 represents the inductor current IL1. Referring to FIGS. 1 and 2, during the on-time of the PWM signal, the power switch 104 is on, and the inductor current IL1 increases. When the PWM signal switches to low, the power switch 104 is turned off, the inductor current IL1 starts to fall down, and the phase node voltage LX is pulled high. Then, when the inductor current IL1 discharges such that the inductor L1 and the parasitic capacitors in the diode D1 and on the phase node LX become an oscillation circuit, LX ringing occurs, as shown by the part circled by the dashed lines in the waveforms 152 and 154 of FIG. 2.
FIG. 3 is a partially enlarged view of the waveforms 152 and 154 in FIG. 2, and FIGS. 4 to 6 illustrate the charge and discharge of the inductor L1 during the LX ringing period. Referring to FIGS. 3 to 6, when the PWM signal switches to low, the power switch 104 is turned off, the inductor current IL1 starts to fall down, and the phase node voltage LX is pulled high. When the inductor current IL1 falls down to zero, as shown at time t1, the phase node voltage LX starts to fall down, and the parasitic capacitors CD and CP in the diode D1 and on the phase node LX will charge the inductor L1, as shown in FIG. 4. At time t2, the phase node voltage LX equals to the input voltage VIN, and the inductor current IL1 reaches its valley. Then, the phase node voltage LX continues falling down until it is lower than the ground potential by a threshold, the body diode 116 of the power switch 104 becomes conductive, as shown at time t3, and the inductor current IL1 begins to flow from ground GND to the inductor L1, as shown in FIG. 5. However, the conductive body diode 116 will clamp the phase node voltage LX at a constant. When the inductor current IL1 increases to be greater than zero, as shown at time t4, the body diode 116 of the power switch 104 is cut off and the inductor L1 starts to discharge to the parasitic capacitors CD and CP, as shown in FIG. 6. Then, the phase node voltage LX starts to raise up until the inductor current IL1 is lower than zero again, as shown at time t5, and thereafter, the process of charge and discharge shown in FIGS. 4 to 6 is repeated. The parasitic charging and discharging the inductor L1 cause the LX ringing, and thus result in LX radiation (EMI) and input/output noise (parasitic coupling).
Therefore, it is desired an anti-ring method and apparatus for an asynchronous boost converter.